In the field of radio frequency (RF) transmitters, mixers are used to convert a low frequency baseband or intermediate frequency up to an RF frequency. Distortion introduced by the mixer may lead to corruption of the fundamental signal, and to unwanted spectral components at other frequencies that may cause involuntary interference to other transmitter units. It is therefore important to minimise the distortion introduced by the mixer.
FIG. 1 illustrates an example of a conventional active mixer structure 100. Active mixers are capable of gain (unlike passive mixers) and use an amplifying device to ensure a required signal strength of the product signal. They also provide good isolation between the input and output ports. Specifically, FIG. 1 illustrates a double balanced mixer. Advantageously, a double balanced mixer has symmetrical paths for both inputs, so that neither of the input signals appears at the output.
The active mixer structure 100 illustrated in FIG. 1 consists of a transconductance stage 110 consisting of two transistor devices Mm1 and Mm2. The transistor devices Mm1,2 are arranged to receive a first (differential) input frequency voltage signal (±)umi and to convert the received frequency voltage signal umi into (differential) input currents io1,2 having a frequency ωb equal to that of the received input frequency voltage signal umi. The active mixer structure 100 illustrated in FIG. 1 further consists of a switching stage 120 consisting of two pairs of switching transistor devices Msw1a,1b and Msw2a,2b The switching transistor devices Msw1a,1b,2a,2b multiply the input currents io1,2 with a second (differential) input frequency voltage signal up,n, having a frequency ωLO, to form differential output currents iM1,2. Isolation transistor devices MB1,2 are provided between the transconductance stage 110 and switching stage 120 to improve the output impedance and isolation, making the transconductance stage 110 more of an ideal transconductor.
In Dimitrios Papadopoulos' 2008 dissertation entitled “A Power Efficient Linear Multi-Mode CMOS radio Transmitter”, Dimitrios Papadopoulos identifies that the distortion introduced by the switching stage 120 of such a double balanced mixer structure may be divided into two types: static overlap distortion (SOD) and dynamic switching distortion (DSD).
FIG. 2 illustrates a first pair of switching transistor devices Msw_1a,1b of the switching stage 120. For SOD, the case without parasitic capacitance Cs at the input node of the switching stage 120 is considered. FIG. 3 illustrates a graph showing the cause of SOD within the first pair of switching transistor devices Msw_1a,1b of the switching stage 120. In an ideal circuit, the switching transistor devices Msw_1a,1b switch instantaneously, with only one switching transistor device Msw_1a,1b within the pair being ‘on’ at any one time. However, in practice the switching transistor devices Msw_1a,1b are not capable of switching instantaneously, resulting in periods of overlap when both switching transistor devices Msw_1a,1b within the pair are ‘on’ simultaneously. As illustrated in FIG. 3, when the switching pair transition from Msw_1a,1b being on to Msw_1a,1b being on, because the input frequency voltage signal up,n has a finite slope, a period of overlap occurs when both of the switching transistor devices Msw_1a,1b are on. When both switches are on during such a period of overlap, the input current io1 is divided between the output currents iMP,N, introducing distortion within the output currents iMP,N in the form of a non-instantaneous, non-linear transition of the output current iM (iM=iMN−iMP) from +iO1 to −iO1.
For DSD, the case with parasitic capacitance Cs at the input node of the switching stage 120 is considered, which represents the gate-source and source-bulk capacitances of the switching transistor devices Msw_1a,1b of the switching stage 120, and the drain-bulk capacitance of the isolation transistor device MB1 (FIG. 1) below the first pair of switching transistor devices Msw_1a,1b. FIG. 4 illustrates graphs showing the cause of DSD within the first pair of switching transistor devices Msw_1a,1b of the switching stage 120. As illustrated in FIG. 4, in order for the switching pair to fully switch from one side to the other, it is unavoidable that a voltage swing us (illustrated in the upper graph of FIG. 4) is generated at the common source node S (FIG. 2) between the switching transistor devices Msw_1a,1b. The voltage us causes a current to flow through the parasitic capacitance Cs that is dependent on the input signal io1 and on device non-linearity, resulting in distortion current flowing through the first pair of switching transistor devices Msw_1a,1b.
In his dissertation, Dimitrios Papadopoulos quantifies the DSD by modelling the pair of switching transistor devices Msw_1a,1b as a single combined source-follower, with transitions happening at twice the switching frequency (2ωLO), illustrated in FIG. 5. As can be seen from this model, the distortion currents that flow through the capacitance Cs at 2ωLO combine with the input current io1, resulting in cross-products that generate distortion currents at harmonics around 2ωLO±n*ωb.
In the mixer (FIG. 1), these distortion current harmonics are mixed with the frequency ωLO of the second (differential) input frequency voltage signal up,n. Accordingly, the distortion currents at harmonics around 2ωLO±n*ωb become down-converted to the frequency of the mixer's fundamental output signal: ωLO±n*ωb.
Thus, there is a need to minimize the effects of SOD and DSD on the output currents iM1,2 of the active mixer structure 100.
In his dissertation, Dimitrios Papadopoulos proposes ‘coping’ with the effects of SOD and DSD by introducing a switch Q between the two switching pairs, which closes for a very short interval during the overlap period of the switching transistor devices Msw_1a,1b. In this manner, the switch Q briefly forces the currents io3 and io4 to be equal, thereby ‘quenching’ the differential current at the output during the overlap period. However, a problem with this solution is in generating the quenching signals, which need to be very concise and timed correctly (the delay needs to be either close to zero, or carefully synchronised) so as to only affect the overlap period. As long as the quenching devices are active, they will short circuit the mixer, and if they are on for longer than necessary it will noticeably deteriorate the mixer operation (reduced gain). Generating such a quenching signal in practice is difficult, and the circuits used to generate such a signal would typically have high power consumption.